Concrete railroad ties are steadily gaining increased use as replacements for wooden ties. Concrete ties have many acknowledged advantages over wooden ties. However, the significant weight differential between concrete and wooden ties can be a determining factor in deciding which will be used in particular geographic areas. Concrete ties are substantially heavier and are consequently more costly to transport than wooden ties.
The most economic method used currently to produce concrete railroad ties is the "long-line" method. In a nutshell, the long-line method involves the use of numerous tie molds joined end-to-end along a stressing bed. This arrangement enables the use of just two wire tensioning devices, an anchor and a tensioner, for tensioning reinforcing wires that span the entire length of the joined molds. With this arrangement, the wires are tensioned only once for each casting cycle, in which hundreds of ties are produced.
In long-line permanent concrete tie making plants, reinforcing wires are disposed along the full length of the frame in sets, one for each column of longitudinally aligned tie forms. The forces required to pre-stress the long reinforcing wires necessitate a carefully engineered and massive stressing frame that, if fairly large numbers of ties are to be produced economically, must span a considerable distance. In permanent tie production facilities, the stressing frame will often span several hundred feet, and be produced as a permanent, in ground structure using several hundred tons of concrete and reinforcing steel in the process. Such stressing frames are not at all practical, if not impossible, to transport from one production site to another.
One solution to the problem of providing stressing frames is to simply construct permanent type stressing frames at the remote site. This is inefficient in that the number of ties that can be produced for one or two projects is significantly less in comparison to what may be produced at permanent facilities on a continuous basis. Once a sufficient number of ties have been produced at the remote site, the poured-in-place stressing frames remain and must be somehow disposed of at substantial cost.
Other solutions offer special concrete tie forms with built-in stressing frame capability. While this eliminates the need to dispose of temporary stressing frames, the forms themselves are heavy and therefore costly to build, transport and handle. Additionally, such "short-line" production methods are affected by the need to repeatedly perform functions for singular, or small numbers of ties, whereas the long-line method requires the same performance only once per stressing frame and casting cycle.
The short-line production method demonstrates significantly lower labor productivity as well as much higher wire wastage in comparison to the long-line method.
A need has remained for portable concrete tie plants that will allow for production of concrete ties at sites geographically closer to the locations at which the produced ties are to be used. Such plants have been produced and used on a limited basis. However, to the best of the applicants' knowledge, until advent of the present invention, no portable concrete tie production facilities have been developed that are economically feasible. This is because economical long-line production techniques have not been adapted to portable concrete tie production facilities or equipment.
A need also remains for a portable long-line tie production structure that is constructed of portable stressing frames that are easy to transport on ordinary roadways, are quick to set up, that emulate permanent long-line tie making facilities to allow for interchangeability of certain equipment and parts, and that can be easily broken down and transported for re-use at another site.
The present system fills the above needs, as will be understood from the following description.